Makes windowserver not send messages during shutdown

services/ui/ws/window_tree.h

Index: services/ui/ws/window_tree.h
diff --git a/services/ui/ws/window_tree.h b/services/ui/ws/window_tree.h
index 245d0c68cba755c3478ed4392f4668c66e8ef137..107e1d1cc010107765f25e99a2df8e52e5666761 100644
--- a/services/ui/ws/window_tree.h
+++ b/services/ui/ws/window_tree.h
@@ -151,6 +151,10 @@ class WindowTree : public mojom::WindowTree,
   WindowServer* window_server() { return window_server_; }
   const WindowServer* window_server() const { return window_server_; }
 
+  // Called from ~WindowServer. Reset WindowTreeClient so that it no longer gets
+  // any messages.
+  void PrepareForWindowServerShutdown();
+
   // Adds a new root to this tree. This is only valid for window managers.
   void AddRootForWindowManager(const ServerWindow* root);